Under certain circumstances, it can be desirable to provide a plurality of power supplies connected in parallel so as to have a common output node for supplying power to a load. If each power supply were to operate independently, however, a disproportionate amount of the current required by the load would tend to be supplied by only one power supply while the other power supplies would supply significantly less current.
Load sharing is a technique which distributes responsibility for supplying current to a load among two or more power supplies having a common output node. Thus, load sharing introduces a degree of interdependency among parallel power supplies. U.S. Pat. No. 5,157,269, issued to Jordan et al., discloses a load current sharing circuit that allows multiple power supplies to be connected in parallel such that each power supply delivers only its proportionate share of the load current. The power supply providing the highest current to the load functions as the master while all the other power supplies function as slaves. Each slave increases its share of load current so as to asymptotically approach the load current supplied by the master within a preset offset, while the load current of the master decreases. The output voltage is regulated with feedback.
This technique has a disadvantage in that when sufficient current is being supplied to the load for maintaining the required output voltage, one or more of the slaves may increase the output voltage in an attempt to match the load current supplied by the master. Thus, if the feedback loop is not properly damped, the output voltage could reach a high level. Also, if any one of the power supplies should fail such that its output current is uncontrolled in that it "runs away," this power supply would become the master and all the others would attempt to follow by providing their maximum outputs. Thus, interdependency among the power supplies introduced by the load sharing technique of U.S. Pat. No. 5,157,269, might result in excessive current.
U.S. Pat. No. 4,717,833 issued to Small, discloses single wire current share paralleling of power supplies where each of a plurality of power supplies produces an output current which is controlled to conform to an average of the currents produced by all the power supplies. This technique has a disadvantage in that if one or more of the power supplies should fail such that it provides more or less than its share of the load current, then the average of the currents changes which could adversely affect the load.
Therefore, what is needed is a load sharing technique which distributes responsibility for supplying current to a load among two or more power supplies having a common output node and which does not suffer from disadvantages of the prior art.